1. Field of the Invention
The present invention relates to a granular colorant which is hardly deteriorated by heat and which has easy handlability, meterability, dispersibility and dilutability, as well as to a method for preparing the same.
2. Description of the Prior Art
As the granular colorant for coloring thermoplastic resins such as polypropylene, a granular colorant of so-called master batch pellets which are prepared by dispersing a dye/pigment of high concentration in a resin is generally known. Since the master batch pellets are prepared by fully melting and kneading thermoplastic resin pellets as melted, kneaded and pelletized and a dye/pigment at a high temperature, these have a drawback that the thermoplastic resin itself is deteriorated by heat and accordingly the properties of the thermoplastic resin as colored with the colorant of the pellets are thereby worsened. In particular, master batch pellets of polypropylene are easily deteriorated by heat because of the influence of thermal hysteresis caused by melting and kneading, and therefore the colorant of master batch pellets of such deteriorated polypropylene noticeably worsens the properties of polypropylene fibers or films as coated therewith. Under the situation, therefore, development of a granular colorant which is not or is hardly deteriorated by heat is strongly desired.
As examples of a granular colorant which is not or is hardly deteriorated by heat, there have heretofore been known a granular colorant prepared by coating the surfaces of polyalkylene resin pellets with a pigment composition comprising a pigment, a low molecular polyalkylene and an antistatic agent (Japanese Patent Publication No. 46-41459) and a granular colorant prepared by further coating the said granular colorant with a low molecular polyalkylene or the like (Japanese Patent Publication No. 56-35688).
However, these granular colorants have some drawbacks. Precisely, the former granular colorant has a poor adhesiveness between the surfaces of the pellets and the pigment composition so that the pigment composition easily peels off from the surfaces of the pellets and the peeled pellets have reduced meterability, dispersibility and dilutability. On the other hand, the latter granular colorant is somewhat improved, as compared with the former granular colorant, since the pigment composition layer is reinforced by the surface coating layer. However, the adhesiveness between the surfaces of the pellets and the pigment composition still is not improved at all, and the improvement of the colorant is neither sufficient nor substantial. Therefore, it is impossible to fully avoid peeling of the pigment composition from the surfaces of the pellets in handling and operation of the colorant, such as conveyance, transportation, metering, blending and so on thereof.